As spelling suggests, the word originally was the opposite of friend and described any hostile enemy (male and female, with abstract noun form feondscipe "fiendship"), but it began to be used in late Old English for "the Devil, Satan" (literally "adversary") as the "enemy of mankind," which shifted its sense to "diabolical person" (early 13c.). The old sense of the word devolved to foe, then to the imported word enemy. For spelling with -ie- see field. Meaning "devotee (of whatever is indicated)," as in dope fiend, is from 1865.